Shielded electrostatic transducer



D. A. STARR, JR

SHIELDED ELEGTROSTATI C TRANSDUCER Filed Oct. 23, 1965 2 Sheets-Sheet l INVENTOR. DAVID A. STARR, JR

Sept. 30, 1969 D. A. STARR, JR 3,470,563

SHIELDED ELECTROSTATIC TRANSDUCER Filed Oct. 23, 1965 2 Sheets-Sheet 2 v LJ N 8 m l 'l x I I l O I 1 Fill I no LL IN VENTOR.

51 DAVID A. STARR, JR. Ll:

United States Patent M 3,470,563 SHIELDED ELECTROSTATIC TRANSDUCER David A. Starr, Jr., Southfield, Mich., assignor to Burroughs Corporation, Detroit, Mich., a corporation of Michigan Filed Oct. 23, 1965, Ser. No. 503,762 Int. Cl. H04r 19/06; G01d 15/06 US. Cl. 346-74 7 Claims ABSTRACT OF THE DISCLOSURE This invention relates to an electrostatic transducer and, more particularly, to an improved, shielded electrostatic recorder and to a method of fabricating the same.

The electrostatic recording process comprises the steps of printing, inking, and fixing. In the printing step, the indicia to be recorded are formed as shaped latent electrostatic images on a surface of high electrical resistivity present on a record sheet or web. In the inking step, these previously deposited charged areas are rendered visible by the application of a finely powdered developing agent or ink which is attracted to the previously charged areas and retained thereon by electrostatic attraction. The third step, which is optional, comprises fixing the inking powder adhering to the latent images on the recording sheet and may comprise the steps of heating the record sheet and subjecting the powdered images thereon to a rolling pressure contact.

Recently it has been discovered that improved recording and greatly simplified apparatus can be produced by employing an electrostatic transducer comprising a plurality of cooperating initiating and printing electrodes. By employing such a multiple element electrostatic transducer, the selection matrix may be placed in the transducer itself thereby greatly simplifying the electronic circuitry required to select predetermined groups of electrodes during the recording operation. For a more complete understanding of the structure and operation of an electrostatic transducer which utilizes cooperating initiating and printing electrodes, reference may be had to Patent No. 3,068,479, granted Dec. 11, 1962, to R. E. Benn et al., which is assigned to the assignee of the present invention.

It is well known that alphanumeric characters may be formed by a plurality of dots as well as by continuous lines. One example of an electrostatic printer which is capable of producing alphanumeric characters on a suita ble recording medium comprises an array of individually selectable initiating electrodes which are cooperable with a lesser number of individually selectable printing electrodes. For example, the electrostatic printing head may comprise a 7 x matrix of initiating pin electrodes which, by cooperating with five printing electrodes, i.e., one for each column of pins, is capable of producing any combination of thirty-five dots arranged in an array of seven rows of dots in one direction and five columns of dots at right angles to the seven rows. The printing head containing the matrix of initiating and printing electrodes is closely spaced from and cooperates with back 3,470,563 Patented Sept. 30, 1969 electrode means to form electric fields in the gap therebetween through which the record medium is fed. The individual initiating electrodes in the printing head are selectively cooperable with the associated printing electrode juxtapositioned therewith to produce an ionizing discharge which, in turn, generates charged particles which aid in the establishment of the electric fields and are deposited in the form of latent electrostatic images on the record medium. Obviously the number of vertical and horizontal rows in a matrix may be varied depending upon the particular printing format desired. The definition of the resulting characters in any event is a function of the spot size.

The size and shape of the electrostatic images formed on a record medium as the result of an electrical discharge between selected initiating and printing electrodes depends on a combination of parameters including the size and shape of the respective electrodes and the polarity of potential applied thereto.

A further improvement in the electrostatic printing art, and particularly in the fabrication of electrostatic print heads is set forth in application for patent of Howell et al., Ser. No. 856,868, now Patent No. 3,235,- 942, filed Dec. 2, 1959, entitled, Electrode Assemblies and Methods of Making the Same, which is also assigned to the assignee of the present invention. Briefly, the improved method of fabricating an electrostatic transducer as set forth in application Ser. No. 856,868 comprises the steps of forming the pin and bar electrodes, using printed circuit techniques on the opposite sides of an insulating substrate or member, and assembling a plurality of such substrates into a laminated structure with the pin and bar electrodes terminating in a substantially common plane. Each lamination includes not only the printed electrodes but also electrical circuit means in the form of leads having terminals for applying to each electrode an appropriate source of electrical potential. The electrical circuit provision for each pin electrode on the substrate includes a printed circuit conductor and a deposited current limiting resistor which is formed very close to the respective pin electrode. The auxiliary or bar electrode for each row or column of pin electrodes is carried on the confronting face of the next adjacent substrate and is insulated from its associated pin electrodes within the laminated structure by a substantially thin interposing layer of electrical insulating material.

In a representative printed circuit head embodying the invention disclosed in the aforesaid Howell et al. application, thirty-five pin electrodes and five associated bar electrodes are positioned within an area of approximately one-sixty-fourth square inch. This small area is dimensionally comparable in size to the striking face of a type bar of a conventional typewriter.

In the operation of printed circuit heads like those disclosed in the previously cited Howell et al. application, the printing results, although generally satisfactory for imparting information, were not of the desired high quality and uniform density. Extra unwanted printed dots would appear on the record medium or often the shade or darkness of the printed characters varied. Considerable investigation was undertaken to ascertain the cause of these undesired and occasionally unreliable printing results and many experimental models of the printing head were made and operated for the purpose of discovering the reason for this undesired printing and for improving the head to avoid such unsatisfactory printing.

After considerable experimentation and study, I determined that a capacitive coupling action was occurring between the electrodes in the print head and was causing either the deposition of unwanted electrostatic charges on the record medium or the non-uniform density of the unwanted electrostatic charges on the record medium. Investigating further, I found that because of the large crosssectional area of the printing electrodes relative to their thickness, the capacitive coupling between pin electrodes on adjacent lamina is much, much higher than the capacitive coupling existing between the electrodes on any given laminations. Moreover, the pin electrodes were common- 1y isolated from their associated pulse drivers by relatively high value current limiting resistors. Due to this high value of resistance in series with the pin electrodes, I found that the operating end of those unselected electrodes which are surrounded by a plurality of selected electrodes would be driven to the selected sense, that is, to assume a printing voltage when none was applied to their respective terminals.

Further investigation revealed that due to the relatively high value of the series pin resistors in the miniaturized printed circuit of the head, the current to a selected pin was less than the critical current required to sustain an electrical discharge, and therefore the electrical are or breakdown which produces the ions for effecting the printing operation is actually a spark discharge of very short duration. This investigation revealed that the capacitor formed by the selected pin electrode and its surrounding environment stored the charge which was involved in the discharge from the electrodes onto the surface of the record member. In other words, each pin resistor was capable of recharging its associated pin electrode back to the operating voltage several times during the duration of a print pulse resulting in several spark or are cycles taking place during the print pulse. As is known, such a spark involves a charge Q, which is a function of the applied voltage and the capacitance of the respective pin electrodes. In my investigation, I found that variation in the lengths of the insulated printed circuit leads for the pin electrodes caused a variance or non-uniformity in the capacitance thereof and that this produced a difference in the discharges from the electrodes and therefore an uneven deposition of the charges on the surface of the record member. The resulting variation in the density of the electrostatically charged areas on the recording medium caused an uneven deposition of the ink for rendering the charged areas visible and the consequent variation in the shade or degree of contrast of the printed matter.

It is therefore an important object of this invention to provide an improved laminated electrostatic print head structure which overcomes the aforesaid difliculties.

It is another important object of this invention to provide an improved electrostatic print head employing printed wiring techniques for forming the working elements which results in reliable, high quality printing,

It is another important object of this invention to provide means for preventing objectionable, uneven deposition of charge on a record medium from various electrodes of an electrostatic print head. i

It is another important object to provide balanced interlamina pin capacitance in an electrostatic print head structure.

It is a further important object of the present invention to eliminate objectionable cross talk between selected and non-selected electrodes in a laminated electrostatic print head.

In accordance with a first aspect of applicants invention, objectionable capacitive cross talk is eliminated in a laminated printed circuit print head by utilizing shaped, electrostatic shielding means insulated from and positioned between cooperating sets of electrodes in the head. In a representative embodiment of this first aspect of applicants invention, the respective pin and bar electrodes may be formed on suitable insulating substrates and subsequently arranged in a laminated structure. The electrostatic shielding means may form part of each substrate, thereby providing in the assembled head structure a plurality of electrostatic shields between the adjacent sets of electrodes carried by the laminations.

In accordance with a second aspect of applicants invention, the objectionable, uneven and non-uniform deposition of charges on.a high resistivity record medium from various pin electrodes of a print head matrix is eliminated by balancing or equalizing the respective areas of the pin electrodes and their associated electrical leads. In a preferred embodiment the width of the conductors associated with the pins is varied as to their length so as to yield substantially equal conductive areas and hence, in combination with the juxtapositioned shielding means, substantially equal capacitances for all pins.

The above listed objects and other aspects of applicants invention may best be understood by referring to the following detailed description in conjunction with the accompanying drawings wherein:

FIG. 1 is a side elevation of a portion of an electrostatic recording station;

FIG. 2 is an enlarged view of the printing face of the matrix printing head illustrated in FIG. 1;

FIG. 3 is a top plane View of a substance of insulating material having pin electrodes and electrical conductors formed on one surface thereof and showing in dotted outline the presence of electrostatic shielding means formed in accordance with the principles of this invention;

FIG. 4 is a top plane view of the opposite side of the substrate of FIG. 3 having formed thereon a bar or printing electrode and electrical conductor therefor and showing in dotted outline the electrostatic shielding means;

FIG. 5 is an exploded perspective view illustrating the various parts of an electrostatic print head formed in accordance with this invention and arranged in the order of their assembly; and

FIG. 6 is an enlarged perspective view of the operating portion of the print head in compactly assembled condition, and partially broken away to show the laminar construction of the head.

Referring now to FIG. 1, there is shown a matrix print head 11 suitable for use in a page printer such as that described in the previously mentioned patent to Robert E. Benn et al. No. 3,068,479. The print face 13 of head 11 is substantially planar in form. Spaced a short distance from face 13 is a backing electrode 15 thus forming an air gap 17 therebetween. A record medium upon which the electrostatic charges are deposited is shown at 18.

The arrangement of the respective initiating or bar electrode 19 and the pin electrodes 21 at face 13 is more clearly shown in the enlarged view in FIG. 2. The thirtyfive pin electrodes 21 may be considered as arranged in horizontal or vertical rows with seven electrodes in each vertical column and five electrodes in each horizontal row. At the face 13 the electrodes 19 and 21 form an ordered array or matrix and terminate in a substantially common plane. The terminals 23 positioned about the sides of head 11 are utilized to selectively apply appropriate printing potentials to the respective printing and initiating electrodes during the printing operation. Each terminal 23 is connected to but one electrode associated conductor as hereinafter described.

Referring now to FIG. 3, there is shown an insulating base or substrate 24, having a top surface 25. The insulating base or substrate 24 is preferably of the glass-epoxy type. However, as will be evident to those skilled in the art, any suitable synthetic or natural electrical insulating material could be employed. Formed on surface 25 of substrate 24 are the pin electrodes 21, terminal pads 23, and individual circuit means for interconnecting the respective electrodes and their associated terminal pads. Each individual circuit means includes a printed circuit conductor or backstripe 29, 29' and a resistor 31 in series therewith. The shaped pin electrodes 21 may be formed on the substrate 24 by any suitable means well known in the art, for example, by a printed circuit technique which involves photoresist and subsequent etching steps. Similarly, the conductors 29, 29 and resistors 31 may be former by any method well known in the art. For example, the conductors may be formed by printed circuit techniques with appropriate sections being completely etched away to accommodate the vapor deposition or painting of a resistive material thereon to form the resistors. While the substrate 24 illustrated in FIG. 3 closely approaches a full scale dimension of one embodiment of applicants invention, the size and shape of the substrate as well as the configurations of the conductors, resistors and pin electrodes may be varied to achieve any desirable configuration or printing format.

Each substrate 24 is provided with an electrostatic shielding provision. This provision may be formed by any method Well known in the art, for example, by cladding the surface 25, as shown in FIG. 4, of the substrate which is opposite to that disclosed in FIG. 3 with a thin metallic sheet and subsequently removing portions thereof by etching. Alternatively, each substrate 24 may be formed of a plurality of very thin laminations made of glass cloth and epoxy, and interleaved among these laminations is a thin metal film 33 constituting the electrostatic shield. Such a laminated form of substrate is compressed to the desired thickness of .0010 to form a fairly dense self-supporting board having the electrostatic shield embedded therein between the opposite surfaces thereof as shown in FIG. 6.

As is shown in the art, the respective conductors 29 may be insulated from one another if the electrode assembly is to operate properly. Because their respective terminal pads 23 are disposed different distances from the pin electrodes 21 and are connected to one another by approximately the shortest routes as shown in FIG. 3, the conductors on several if not all of the substrates will be of varying lengths. Similarly, as shown in FIG. 3, the conductive portions 29' connecting the other end of the resistors 31 to the respective pin electrodes 21 will likewise be of varying lengths. In order to balance the capacitance of the different pin electrodes and their surrounding environment, it was found that the areas of the deposited electrical conductors 29, and particularly the conductive portions 29' thereof, should be constant and independent of the length thereof. Therefore, in order to eliminate objectionable capacitance cross talk, the capacitance of all the pin electrodes and the area of their respective conductors 29 and 29 should be held constant regardless of any variation in their respective lengths.

Accordingly, in one embodiment of applicants invention, the conductive area of the respective variable length conductors 29 and 29' are equalized by varying the width of the respective conductors inversely in proportion to their lengths such that the length-width product is a constant.' In accordance with another embodiment of applicants invention, the balancing of the conductive areas of the variable length conductors 29 and 29 may be accomplished by employing equal width conductors 29 in conjunction with graduated area conductive tabs 30 and 30' as shown in FIG. 3. The respective areas of the conductive tabs 30 and 30', which are utilized to form the electrical connections at the respective ends of resistors 31, are chosen such that the product of length times the width of the respective conductors 29 and 29' plus the area of the associated tabs 30, 30 and associated pin electrode 21 equals a constant.

As previously mentioned, the electrostatic shielding means 33 is formed as a metal film embedded within each substrate. As will be evident to those skilled in the art, the shielding means 33 of each substrate may be of any general configuration but preferably comprises a thin shaped metallic sheet which substantially covers an area occupied by the conductors 29, 29', resistors 31, and pin electrodes 21 as shown by its dotted outline in FIGS. 3 and 4.

Referring now to FIG. 4, there is shown the opposite side 25 of the base or substrate 24 illustrated in FIG. 3, having formed on one side thereof the extended printing electrode 19 and its associated conductor 45, resistor 47 and terminal tab 49. The electrode 19 and its associated conductor 45, resistor 47 and terminal tab 49 may be formed by any method well known in the art, for example, the hereinabove mentioned printed circuit techniques.

In a print head embodying applicants invention as illustrated in FIG. 4, the extended printing electrode 19 would be cooperatively juxtapositioned with a plurality of pin electrodes formed on an adjacent lamination of an assembled laminated structure and the metallic film 33 associated with each lamination would form an electrostatic shield for the pin electrodes on the adjacent lamina. As for the substrates 24 which constitute the outermost lamina in a transducer structure, their respective electrostatic shielding films 33 also form electrostatic shields for isolating the print head structure from external disturbances.

Referring now to FIG. 5, there is shown an exploded view illustrating details of construction of a laminated print head which embodies the principles of applicants invention. As illustrated, laminae L1 through L6 have formed on side 25 thereof the respective pin electrodes 21 and their associated terminal pads 23, conductors 29 and 29', and resistors 31. The electrostatic shielding means 33, only that on L-l being indicated in dotted outline for purposes of illustration, would be carried by each lamination. In the assembled laminated structure each shielding means 33 electrostatically prevents the pin electrodes 21 and their associated electrical circuits on one lamina from affecting like elements on the immediately adjacent laminations. As would be evident to those skilled in the art, the metallic electrostatic shielding means 33 must be insulated from the printed conductors and electrodes of the next adjacent lamina. Moreover, such shielding means should be grounded. These results are accomplished by embedding each shield between the opposite surfaces of each substrate 24 and by providing an extension 51 on each shield which in the ultimate assembly of the head reaches the exterior of the head for connection to a grounded conductor. However, the exact method selected to insulate the electrostatic shielding means 33 from the electrodes 21 and conductors 29 and 28 of the adjacent lamina may vary depending upon the method utilized to position the bar electrode bearing lamina in the head structure. As earlier suggested herein, the metallic films making up the electrostatic shields 33 could be deposited or otherwise formed on one surface of a substrate bearing either the bar electrode 19 or the pin electrodes 21 on the opposite surface of the same substrate. However, such an arrangement would increase the number of laminations in the head assembly.

As disclosed in the aforementioned Howell et al. patent application, Ser. No. 856,868, the pin electrodes 21 are deposited or otherwise formed on one corresponding face of each substrate 24 and in the sam location thereof so that they terminate in the matrix of rows and columns shown in FIG. 2. However, the input ends of the conductors 29 leading to the pin electrodes 21 are arranged to approach different edge portions of the substrates as shown in the exploded view of FIG. 5 in order that individual electrical connections may be made to these conductors from an external control source. As for the bar electrodes 19, these electrodes are deposited or otherwise formed on the opposite faces of each substrate and are located substantially coextensive with the space occupied by the pin electrodes 21 carried by the same substrate. However, as described in the aforementioned Howell et al. application, the pin and bar electrodes on the same substrate do not electrically cooperate with one another to perform the printing operation. Instead the pin electrodes on one substrate cooperate with the bar electrode on the confronting face of the adjacent substrate to produce the desired electrical fields for forming the electrostatic charges on the recording medium. To accomplish this operation, it is necessary to insulate the confronting electrodes from one another but in such a manner that the desired minimum spacing of these electrodes is obtained for producing the electrical fields. For this purpose, there is provided a thin insulating laminate or shim between the confronting bar and pin electrodes, such as indicated at 53 in FIG. 5. As described in the aforesaid Howell et al. patent application, this shim of insulating material is of substantially less thickness than th thickness of the substrates 24 constituting the laminations L1 to L-6 of FIG. 5.

The exploded assembly of elements shown in FIG. 5 is compactly compressed together to form a laminated structure in which the pin electrodes on the face of each substrate will cooperate electrically with the bar electrode on the confronting face of the next substrate. This relationship of the parts is disclosed in FIG. 6. Additionally, FIG. 6 shows the preferred location of the metallic film 33 forming the electrostatic shield of each substrate; it being approximately midway between the opposite faces of each substrate.

In this assembled condition, the several laminations of the print head are bonded into a unitary structure by means well known in the art, for example, by an epoxy glue. The print head thus formed will then have the edge toward which the pin and bar electrodes project ground or cut away such that the electrodes terminate in exposed condition in a common plane similar to that shown in FIG. 2. Additionally, the edges of the laminated assembly toward which the terminal ends of the conductors 29 extend are ground or cut away and then they may be notched as shown in FIG. 1 to expose each terminal pad 23 in a notch in order to facilitate the attachment of an electrical connection thereto. For example, as shown by comparison of FIGS. 1 and 5, one end member 29' of each array of conductors 29 on each substrate is shown in dotted outline in FIG. 1 as having its respective terminal pad exposed in a notch of the printing assembly. It is understood that in the finally assembled head all the conductors 29 on each substrate will have their terminal pads 23 exposed in this manner in individual notches of the assembly.

For the operation of this printing head, the printing or bar electrodes are all connected to a common conductor leading to the print head, and for the purpose of this connection the conductors 49 for the bar electrodes have their respective terminal pads in alignment so that as indicated in FIG. 1 one notch in the head assembly is common to all of these pads. Similarly, the corresponding extensions 51 of the electrostatic shields 33 are in alignment and terminate in the same notch of the head assembly for connection to a common conductor which either may be grounded or employ as a reference potential the potential of the unselected, unenergized pin electrodes. Applicants electrostatic shielding means 33 performs a dual function in th laminated head structure. As hereinabove stated, a nonselected electrode which is surrounded by a plurality of selected energized electrodes tends during the printing operation to be driven by capacitive coupling to the selected potential level. In practice it is not practical to lower the value of the series resistors to a point wher this capacitively induced cross talk would be eliminated by merely providing a low resistance path to the electrode associated bias supply since this would permit an inordinately large value of current to flow in a selected electrode circuit. Applicant has discovered that capacitive cross talk was causing this unreliable printing and that by providing an electrostatic shield between the electrodes on adjacent lamina the effects of capacitively induced cross talk could be substantially eliminated. Thus by replacing the capacitive coupling between electrodes on adjacent lamina with a slightly larger passive capacitive loading, i.e., the electrostatic shield, the undesirable effects of electrode to electrode coupling have been successfully eliminated.

In operation, upon simultaneous application of a print initiating pulse and a print pulse to selected pin and bar electrodes respectively, the capacitance of a selected pin electrode acts as a charge pump. Therefore, by balancing the area of the respective pin electrodes and their associated conductors in a matrix array so as to yield substantially equal cross sectional areas, the capacitance of the respective pin electrodes is effectively equalized thereby equalizing the charge associated with the spark discharge of each electrode. Further, ap'plicants shielded head structure, by increasing and equalizing the effective capacitance of each of the pin electrodes, has increased the quantum of charge associated with each spark discharge and therefore a greater amount of charge may be deposited for a given number of spark discharge cycles. Thus an electrostatic transducer employing the balanced and shielded head structure of this invention enables the employment of a shorter duration print initiating pulse.

The foregoing description is by way of illustration only and will be evident to those skilled in the art. The invention is adaptable for fabricating improved electrostatic print heads having a wide variety of laminated configurations without departing from the spirit of the disclosed invention. It is, therefore, the intention to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. An electrode assembly comprising a lamination of a plurality of insulating layers each formed with a plurality of closely spaced substantially parallel electrodes on one surface thereof each having an electrical lead coupled thereto, the electrodes of all the insulating layers being in alignment with the laminated assembly and terminating along an end portion thereof, said laminated assembly having an edge portion thereof formed with a flat face so that the electrodes terminate in the face as a matrix of exposed coplanar pin electrodes and a thin metallic layer associated with each lamination substantially coextensive with the entire surface portion of said electrodes and said leads and extending substantially parallel to the electrode bearing surface thereof, said metallic layers being electrically insulated from said electrodes and said leads and serving as electrostatic shields in the assembly.

2. An electrode assembly comprising a lamination of a plurality of insulating layers; each layer having formed on one surface thereof a plurality of closely spaced substantially mutually-parallel electrodes each having an electrical lead containing a resistor, the electrodes formed on the insulating layers being in alignment in the laminated assembly and terminating along a fiat faced portion thereof, the terminal ends of said electrodes being exposed in said flat face to form a matrix of pin electrodes, and a thin metallic film embedded within each layer and projecting over a substantial portion of the surface area of the electrodes and the electrical leads, said film being electrically insulated from said electrodes and said leads and serving as an electrostatic shield in the assembly.

3. In an electrode assembly for an electrostatic printing head including an insulating layer having formed on one surface thereof a plurality of closely spaced relatively fiat electrodes each having an electrical lead containing a resistor and having formed on its opposite surface an auxiliary printing electrode registering with said closely spaced electrodes but separated therefrom by the thickness of the layer, all of the electrodes terminating in a common plane on one face of the assembly to form a row of pin electrodes spaced from a printing auxiliary electrode by the thickness of the insulating layer, the improvement comprising the incorporation of a thin metallic film in each layer spaced from the opposite surfaces'thereof.

4. An electrostatic printing head comprising a laminated assembly of electrical insulating layers of like thickness, each of said layers having formed on one surface thereof a group of electrodes and an electrical lead for each such electrode containing a resistor therein, each group of said electrodes terminating along a corresponding edge portion of its respective layer and being exposed thereat, each of said insulating layers having formed on its opposite surface an auxiliary electrode which is in registration with sad group of electrodes but separated therefrom by the thickness of the layer, the corresponding electrodes being in registration throughout the laminated assembly and all the terminal ends of the electrodes lying in a common plane on one face of the laminated assembly to form a matrix of rows of electrode groups having the auxiliary electrodes interleaved therebetween, and a metallic film associated with each layer in electrically insulated relation to the electrodes, said films being relatively thin compared to the thickness of the layers and serving as electrostatic shields in the printing head.

5. An electrostatic printing head assembly comprising, in combination, a plurality of substrates composed of electrical insulating material and having the same thickness, each said substrate having one or more first electrical conductors on one surface thereof, terminating at a corresponding edge portion of each substrate, and each said substrate having a second conductor on the opposite surface of the said substrate and terminating at said edge portion thereof, said substrates being compactly assembled in homologus relation to one another with said edge portions thereof in alignment, and an electrical conducting film of substantially less thickness than the thickness of the substrates embedded between the opposite surfaces of each of said substrates, said films being substantially coextensive with the areas of the substrates occupied by the conductors on the surfaces thereof and serving as electrostatic shields.

'6. An electrostatic printing head assembly, comprising, in combination, a laminated assembly of electrical insulating layers of substantially the same thickness, a plurality of thin but relatively wide first conductors carried on one surface of each insulating layer and terminating proximate to an edge portion thereof, a second conductor carried on the opposite surface of each insulating layer and extending along said edge portion in registration with the terminal ends of said edge portion in registration with the terminal ends of said first conductors and being substantially coextensive with the span of space occupied by the first conductors, a thin metallic layer substantially coextensive with the entire surface of said first conductors and insulated therefrom embedded in each of said insulating layers, certain of the first conductors being unequal in length and having compensating variations in width to present substantially equal conductive areas to the metallic layer among the first conductors thereby substantially equalizing the capacitive coupling action between the first conductors and the metallic layer.

7. An electrostatic printing head assembly, comprising, in combination, a plurality of substrates composed of electrical insulating material and of like thickness, and compactly assembled together in side-by-side relationship, each said substrate having a set of thin but relatively wide electrical conductors on a surface portion of each substrate and terminating at an edge of the substrate, a thin metallic film substantially coextensive with the entire sur- References Cited UNITED STATES PATENTS 3,267,485 8/1966 Howell 34674 BERNARD KONICK, Primary Examiner L. J. SCHROEDER, Assistant Examiner US. Cl. X.R.

Egg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 470 563 Dated September 30 1969 Inventor-(s) David A Starr Jr.

It is certified that error appears in the above-identified patent hat said Letters Patent are hereby corrected as shown below:

Column 4, line 20, "substance" should be -substrate Column 4, line 75, "former should be -formed-; Column 6, line 42, "28 should be 29-; Column 9, line 4, "sad" should be -said-; Column 10, lines 2 & 3, delete "edge portion in registration with the terminal ends of said" Column 10,

line 26, layer" should be --film.

and t SIGNED A'ND SEALED JUN 2 .1970

6 Attcat:

Edward M- Flewhm van-1.1m r. mum, JR. Awning Officer Commissioner of Pattlg" 

